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I I I I I I I \1 I I I I I I I I I I I This data was used to develop floodplain limilS established by FEMA. Currently, FEMA has completed the preliminary revised floodplain delineations. The maximum lOO-year flood level is estimated to be at elevation 4816.9 USGS (104.5 relative). The channel bed is at elevation 4804.9 (92.5 relative), which makes a depth of 12 feet. The capacity of the existing structure was determined so that proposed improvements would either match or exceed the operation of the existing structure. The existing structures capacity with all of the gates fully opened was detennined to be approximately 3,170 cfs. This is based on the maximum elevation that can be achieved before the right bank is overtopped at elevation 99.5. At the point of overtopping, the right (west) floodplain begins to carry a considerable amount of flow. Based on the Corp of Engineers hydrology infonnation, the existing structure cannot pass the 10-year stonn in the channel. At the 100-year level, the entire structure would be overtopped by approximately 3.5 feet. The gated structure has experienced considerable operational problems during high flows from debris accumulation, which amplifies the structural stability problems. A.8. STRUCTURE PLANS 1. Alternatives. Several alternatives were initially reviewed and discussed for the rehabilita- tion of the diversion structure. After eliIninating some of the initial alternatives and combining others, three fmal alternatives were:: chosen for further review. The details of the three alternatives pursued in this study are described below and are shown on Figure V-2. Alternative No. 1. The minimum cost alternative considered in this study was to patch the existing structure and fix the gates as needed to make them more convenient to operate. This alternative would not significantly increase the structure's overall stability but would slow the deterioration of the structure and increase the expected life span of the structure. Greater ease in operating the gates may reduce the likelihood of problems during a flood and may save the structure in a marginal condition. Repair of the structure would include the following items: 1. Place a new concrete slab over the entire base slab. This would repair and replace the portions of the base slab that are eroded, broken, and spalled and would be designed to fix the pie::r to base slab joint. 2. Grout the voids beneath the base slab where the underseepage has eroded material from the foundation. 3. Patch the piers as needed to reduce further freeze-thaw deterioration. 4. Place a new concrete apron downstream of the structure and energy dissipation blocks. PAGE 10 . CJaprr:rV